This invention relates to products made from polyolefins and process additives used in connection therewith. In particular, this invention relates to extrudable compositions for producing polyolefin films and to expandable compositions for producing polyolefin foam products.
Various processes and equipment for extrusion foaming of thermoplastic resins have been used for many years. Generally, solid pellets of thermoplastic resin are fed through a hopper to a melting zone in which the resin is melted, or plasticized, to form a flowable thermoplastic mass. The plasticized thermoplastic mass generally is then transported to a mixing zone where the thermoplastic mass is thoroughly mixed with a blowing agent under pressure for subsequent cooling and substantially free expansion of the resin to form a foam. The blowing agent expands the molten mass to form the cells of the foam and the thermoplastic foam is cooled.
The blowing agent gradually diffuses from the cells of the foam and is eventually replaced by air diffusing into the cells. Diffusivity of the blowing agent and the selection and use of appropriate blowing agents are important aspects of foam manufacture. If the diffusivity of a blowing agent out of the cells of a foam is too fast compared to the diffusivity of air, so that the blowing agent is not replaced by air as it escapes, then the foam typically collapses and is said to have dimensional stability problems. For many years, chlorofluorocarbons (xe2x80x9cCFCsxe2x80x9d) were used that had excellent diffusivity characteristics and resulted in high quality, dimensionally stable foam products. However, CFCs are no longer acceptable as blowing agents because of global regulations prohibiting their use.
Other compounds, including lower hydrocarbons, alcohols and ketones, various hydrofluorocarbons, and inert gases have been proposed as alternative blowing agents to CFCs. Some of these compounds diffuse out of the foam cells at a rate that reduces dimensional stability and can result in collapse of the foam cells. Aging modifiers have been developed for incorporation into polyolefin resins that slow the diffusion of selected blowing agents out of the polyolefin foam cells. These aging modifiers are sometimes referred to as permeability modifiers or stability control agents. Among the various aging modifiers that have been proposed are the saturated higher fatty acid amides, saturated higher aliphatic amines, esters of saturated higher fatty acids, copolymers of ethylene and unsaturated carboxylic acids, and others.
One of the more widely used aging modifiers for polyolefin foams is glycerol monostearate. Glycerol monostearate is also added to resins for extrusion to form films. Glycerol monostearate is also called GMS, glyceryl monostearate, and monostearin. GMS is a monoglyceride and is an ester of stearic acid, which is C18 acid, and the trihydric alcohol glycerol. GMS is a pure white or cream colored and wax-like solid.
GMS and other similar aging modifiers are thought to coat the walls of the foam cells to slow the blowing agent gas from escaping and thereby to prevent collapse of the cells of the foam. However, these aging modifiers can leave a grease-like residue on the surface of the foam that can be transferred to objects that come into contact with the foam. The transfer of this grease-like residue to certain substrates is problematic and is particularly undesirable on optical products and high gloss finishes.
GMS also tends to slow the rate of blowing agent diffusion from the cells to the point that some residual blowing agent is maintained in the cells for an undesirably long period of time after manufacture. Too slow a rate of blowing agent diffusion from the cells of the foam can result in dimensional stability problems. Foam rolls can become tight in storage.
It would be desirable to provide a foam product in which an acceptable blowing agent would escape from the cells of the fresh foam at a rate that more closely matches the rate of air entering the cells of the foam to substantially eliminate dimensional changes in the foam upon aging. It would also be desirable to provide foam and other products, including films, in which transfer of grease-like residue to substrates is reduced.
The invention relates to process additives for polyolefin products that can substantially reduce greasy deposits on substrates and, in foams, promotes an increased rate of escape of blowing agent from the cells as compared to glycerol monostearate (GMS) alone. The process additives are particularly useful for producing stable foams from carbon dioxide based blowing agents and reducing the aging time for these foams. The process additive comprises at least one fatty acid N-aliphatic alcohol amide and can be a secondary or tertiary amide.
An ester of a long chain fatty acid with a polyhydric alcohol, including GMS, can be used with the amide, while still achieving the benefits of the invention. The two components can be added to the polyolefin resin either in admixture or separately. The amide and the ester have been observed to exhibit a substantially single differential scanning calorimetry (DSC) melting point that varies linearly depending on the relative amounts of amide and ester, thus signifying some chemical compatibility. It has also been observed that the transfer of grease-like residue is more substantially reduced when both the ester and the amide are present in the polyolefin resin than when the amide is used in the absence of the ester.
Typically the ester and the amide are present in the process additive in a ratio of the ester to the amide of from about 0:1 to 10:1. The polyolefin and process additive typically are present in a ratio of from about 100:0.01 to 100:5, respectively.
The fatty acid N-aliphatic alcohol amide is a secondary or tertiary amide having the formula Rxe2x80x94CON(Rxe2x80x2)Rxe2x80x3. R is a fatty hydrocarbon radical having from about 8 to 30 carbons. Rxe2x80x2 typically is hydrogen. Rxe2x80x2 can also be an alkyl radical of from about 1 to 6 carbons or an alkyl alcohol radical of from about 1 to 6 carbons. Rxe2x80x3 is an alkyl alcohol fragment of from about 1 to 6 carbons. The alkyl alcohol fragments can be monohydric or polyhydric. Secondary fatty monoalkanolamides, in which Rxe2x80x2 is hydrogen and which have the general formula RCONHRxe2x80x3, are particularly useful process additives.
One method of preparing the amide is to react a fatty acid with an alkanolamine, especially a monoalcohol amine. The amine group of the alkanolamine is substituted for the hydroxyl moiety of the fatty acid carboxyl group to form a molecule having a fatty acid amide moiety and an aliphatic alcohol moiety characterized by the attachment of the alcohol moiety to an amide nitrogen. The fatty acid is typically coconut acid, lauric acid, stearic acid, palmitic acid, or oleic acid. The alkanolamine is typically monoethanolamine (MEA), monoisopropanol amine (MIPA), n-propanolamine, betapropanolamine, or 2,3-propanediol amine. An example is the fatty monoalkanolamide stearyl monoethanolamide, which has the formula CH3(CH2)16CONHCH2CH2OH and is sometimes called stearamide MEA. Another example, in which the alcohol is dihydric, is stearyl 2,3-dihydroxy propyl amide, which is also called stearamide 2,3-propanediol and which has the formula CH3(CH2)16CONHCH2CH(OH)CH2OH.
The invention includes polyolefinic compositions that are films or foams and compositions for preparing polyolefin foam products in which the above process additive is incorporated. Typically, the resin is selected from the group consisting of ethylene or propylene homopolymers and copolymers of ethylene or propylene and a copolymerizable monomer. Stable foams having a relatively short aging time can be prepared from carbon dioxide blowing agents, including blends of carbon dioxide with hydrocarbons, including the volatile hydrocarbons.
The invention includes a polyolefin foam product that comprises an expanded polyolefin made from the foamable composition described above. Grease-like transfer to substrates is substantially reduced as compared to GMS alone. Aging time is significantly reduced without loss of stability. Sheet products are useful because sheet products are most typically used to protect high gloss finishes, optical products, and the like. However, the invention is also applicable to plank foam products and to films.
Thus the invention provides a process additive that can function as an effective aging modifier, a foamable composition containing the process additive, and a polyolefinic foam made from the foamable composition. The foam exhibits reduced grease-like transfer to substrates and is dimensionally stable. Aging time is significantly improved and can be reduced by as much as half or more. Carbon dioxide blowing agents can be used. The invention also provides extrudable composititons and films made from them that incorporate a process additive of the invention.